The RBMK reactor

• The RBMK is an unusual reactor design, one of two to emerge in the Soviet Union.
• The design has core characteristics which make it prone to power surges.
• It has no full containment structure.
• Major modifications have been made to RBMK reactors still operating.

The combination of graphite moderator and water coolant is found in no other power reactors. The design characteristics of the reactor mean that it is unstable at low power levels, and this was shown in the Chernobyl accident. The instability was due primarily to control rod design and a positive void coefficient. A number of significant design changes have now been made to address these problems.

Features of the RBMK
Fuel: Pellets of slightly-enriched uranium oxide are enclosed in a zircaloy tube 3.65m long, forming a fuel rod. A set of 18 fuel rods is arranged cylindrically in a carriage to form a fuel assembly. Two of these end on end occupy each pressure tube.

Pressure tubes: Within the reactor each fuel assembly is positioned in its own vertical pressure tube or channel about 7 m long.. Each channel is individually cooled by pressurised water which is allowed to boil in the tube and emerges at about 290° C.

Refuelling: When fuel channels are isolated, these fuel assemblies can be lifted into and out of the reactor, allowing fuel replenishment while the reactor is in operation.

Graphite moderator: A series of graphite blocks surround, and hence separate, the pressure tubes. They act as a moderator to slow down the neutrons released during fission so that a continuous fission chain reaction can be maintained. Conductance of heat between the blocks is enhanced by a mixture of helium and nitrogen gas.

Control rods: Boron carbide control rods absorb neutrons to control the rate of fission. A few short rods, inserted upwards from the bottom of the core, even the distribution of power across the reactor. The main control rods are inserted from the top down and provide automatic, manual or emergency control. The automatic rods are regulated by feedback from in-core detectors. If there is a deviation from normal operating parameters (e.g. increased reactor power level), the rods can be dropped into the core to reduce or stop reactor activity. A number of rods normally remain in the core during operation.

Coolant: Two separate water coolant systems each with four pumps circulate water through the pressure tubes to remove most of the heat from fission. There is also an emergency core cooling system which will come into operation if either coolant circuit is interrupted.

Steam separator: Steam from the heated coolant is fed to turbines to produce electricity in the generator. The steam is then condensed and fed back into the circulating coolant.

Containment: There is no secure containment in the sense accepted in the West. The reactor core is located in a concrete lined cavity that acts as a radiation shield. The upper shield or pile cap above the core, is made of steel and supports the fuel assemblies. The steam separators of the coolant systems are housed in their own concrete shields.

Immediate Safety Changes

After the accident at Chernobyl unit 4, the primary concern was to reduce the positive void coefficient. All operating RBMK reactors, in the former Soviet Union therefore, had the following changes implemented to improve operating safety:

• To improve the operational reactivity margin, the effective number of manual control rods was increased from 30 to 45.
• The installation of 80 additional absorbers in the core to inhibit operation at low power.
• An increase in fuel enrichment from 2% to 2.4% to maintain fuel burnup with an increase in neutron absorption (ie less reliance on cooling water for this).

These factors have reduced the positive void coefficient from +4.5 b to +0.7 b , eliminating the possibility of power excursion. Beta (b ) is the delayed neutron fraction, which is neutrons emitted from each fission with a measurable time delay.

The next consideration was to reduce the time taken to shut the reactor down and eliminate the positive void reactivity. Improvements include:

• Scram (rapid shut down) rod insertion time cut from 18 to 12 seconds.
• The redesign of control rods.
• The installation of a fast scram system.
• Precautions against unauthorised access to emergency safety systems.

Redesign of the control rods.

179 of 211 control rods are inserted into the core from the top. To improve their effectiveness, they are equipped with "riders" fixed to their bottom end but with a gap between the rider and the bottom tip of the control rod. Approximately 1.0m water columns remained under and above it. When the control rod is in its uppermost position, the rider is in the control rod cooling tube within the fuelled region of the core. The rider being made substantially of graphite, is almost transparent to neutrons, while water, which would occupy the tube otherwise, plays as an absorber. When the reactor is "poisoned" with xenon and with partially inserted control rods, the major part of the power is produced within the lower region of the core. This means that when the rod started to move down from its uppermost position, the rider removed water from the lower part, causing an increase in reactivity and hence in power.

Longer-term modifications

In addition to the safety changes, it was decided that RBMKs should be modified more fundamentally. Chernobyl unit 1 was relicensed for operation in October 1995, following extensive maintenance which included the removal of some fuel channels to evaluate the metal and some backfitting as follows. The modification process consists of:

• Replacement of the fuel channels at all units except Smolensk-3.
• Replacement of the group distribution headers and addition of check valves.
• Improvements to the emergency core cooling systems.
• Improvements of the reactor cavity over-pressure protection systems.
• Replacement of the process computer, SKALA.

There are currently 13 RBMKs in the world, in Russia and Lithuania, and one more under construction in Russia (Kursk-5) due for commercial operation in 2003. The reactors have a net generating capacity of 12,545 MWe. All operating RBMKs began operation between 1973 (Leningrad-1) and 1990 (Smolensk-3). There are currently three distinct generations of reactors having significant differences with respect to their safety design features: ·

• The four first-generation units are Leningrad-1 and -2, and Kursk-1 and -2. They were designed and brought on line in the early-to-mid 1970s, before new standards on the design and construction of nuclear power plants (OPB-82) were introduced in the Soviet Union.
• Second-generation RBMKs, brought on line since the late 1970s and early 1980s include Leningrad-3 and -4; Kursk-3, and -4; Ignalina-1; and Smolensk-1 and -2. Ignalina-2 contains safety features beyond those of other second generation units. These units conform to updated standards issued in 1982.
• After the Chernobyl accident, Soviet safety standards were revised again (OPB-88). One RBMK (Smolensk-3) has been built to these third-generation standards. Additional design changes now are being incorporated in the construction of Kursk-5.

The 'operating till' dates in the following table are the scheduled shut down for these plants. However, Russia has adopted a policy of extending plant life, aiming to add a further 10 years to their operation. Lithuania, on the other hand, is likely to close Ignalina early as a condition for entry into the European Union. Russia's long-term plans include the possibility of replacing the Leningrad units, at the end of their extended service life, by new MKER-1000 units. These are a modification of the RBMK design. The main differences are in the spacing of the graphite lattice in the core and the incorporation of passive safety systems.